1. Field of the Invention
The present invention relates to an altitude detecting apparatus for a flying object and more particularly to an altitude detecting apparatus for a flying object flying at a relatively low altitude.
2. Prior Arts
When a flying object flies at a low altitude, or when it makes a take off or a landing, an altimeter is an essential apparatus. In recent years, such a technique as spraying agricultural chemical by means of an unmanned helicopter has come into a stage of practical use. In this case, there are two ways in controlling the unmanned helicopter. One is a remote control in which an operator employs a radio-control of the helicopter by visual observation and the other is an autonomous control in which the helicopter flies or makes a landing and the like autonomously without the help of an operator. In the former case, it needs a high skill to continue to monitor an altitude of the helicopter by eye observation or to make a landing safely from a remote place. Further, since the primary object of using the unmanned helicopter is to save labor, it is desirable that the unmanned helicopter is autonomously controlled without the help of a skilled operator. Therefore, for this type of unmanned helicopter, an altimeter offering high precision altitude data is indispensable.
There are many kinds of altimeters such as an altimeter in which an altitude is obtained by transforming the difference of atmospheric pressure into altitude, an altimeter in which an altitude is calculated from a reflecting time when laser beam, radio wave, supersonic wave and like is transmitted to the ground surface, an altimeter in which an altitude is found based on positional information transmitted from positioning satellites such as D-GPS, an altimeter in which an altitude is obtained from acceleration and other kinds. Taking a measuring accuracy of ground altitude, a reflecting characteristic of object on ground, an environment under which the altimeter is used and the like into consideration, these altimeters have merits and demerits respectively. Few altimeters satisfy conditions suitable to an altimeter for a flying object flying at low altitude.
In order to satisfy such conditions, it is particularly important that the altimeter is free from the reflecting characteristic of an object on ground. With these in mind, a technique in which an altitude is measured by processing images taken by cameras from the flying object, is most promising. As an example of a technique of detecting altitude by images, there is a prior art disclosed in Japanese Patent Application No. Toku-Kai-Shou 62-88914. In this prior art, an altitude is obtained based on the comparison of the movement of an object in an image area with an actual distance to that area.
However, according to this prior art of measuring altitude based on image, since the actual distance to the area must be known, a map including the imaged area is needed and also a specified mark must be imaged in the area. Further, an angle of depression with respect to the center of field of view must be known and this angle of depression is measured indirectly by a gyro compass and the like, thereby the measuring accuracy is exacerbated.